I’d say it was a mistake in Designer. The underlying API supports an expression for that property value, so it is only the Designer UI that is not allowing you to type or pick an expression.
If you require a fix, then your best course of action would be to open an SR so it can be formally tracked. Could you please help by filing an SI (Support Incident) with GS (Global Support)? Please include all the necessary information so that GS may record and also review for any other issues that match. If GS needs to create an iTrac, then whichever R&D engineer handles this case will be expecting a clear description of the issue, logs and any other details in order to efficiently be able to help. Providing this info in the beginning will make the process go faster.
#webMethods#MWS-CAF-Task-Engine#webMethods-BPMS